OLYMPIC NATIONAL PARK — When the Elwha and Glines Canyon dams are gone, what will happen to the 25 million cubic yards of sediment that was trapped behind them?
The U.S. Geological Survey estimates that 9 million to 10.5 million cubic yards of silt, sand, clay, cobbles and gravel will be carried downstream to the Strait of Juan de Fuca in the largest controlled release of sediment into a marine environment in history.
The rest will be covered by vegetation and remain where Lake Aldwell and Lake Mills — the manmade reservoirs created by the dams — once existed.
Within five years of dam removal, the effects of the dams removal will be seen at the mouth of the Elwha River and at nearby beaches, including Ediz Hook in Port Angeles.
“The Elwha nearshore is very severely sediment-starved, and we see that in the ecological function,” said Anne Shaffer, coordinator of the Elwha Nearshore Consortium, a group of citizens, scientists and managers dedicated to understanding and promoting the nearshore restoration of the Elwha dams removal.
Nearshore habitat
Shaffer described nearshore habitat as “critical for a number of species,” including salmon.
When the 108-foot Elwha Dam and 210-foot Glines Canyon Dam were built nearly a century ago, sediment that had moved down the river since the last ice age was suddenly blocked.
By removing the dams, the Elwha nearshore will regain some 30 percent of its total sediment budget in what Shaffer described as “a golden opportunity to realize partial restoration.”
The other sediment that used to feed Ediz Hook, a huge sand spit that forms Port Angeles Harbor, came from the feeder bluff between the mouth of the river and the Hook.
The feeder bluff was armored from natural wave action to protect utility pipes, and the city of Port Angeles has no plans to remove the armor, Shaffer said.
To put the sediment blocked by the dams into perspective, the USGS said it could fill a football field to the height of 11 Empire State Buildings.
The USGS said half to two-thirds of that sediment is fine-grained silt, clay and sand.
The rest is coarse-grained cobbles and gravels.
Ediz Hook eroding
The U.S. Army Corps of Engineers said Ediz Hook has been thinned by 35 percent since the dams were built.
The hook is eroding at a rate of 4 to 5 feet per year.
The Bureau of Reclamation said that upwards of 2 million cubic yards of sediment will find its way to Ediz Hook.
Meanwhile, the Corps will restore the Hook’s seaward shore later this year with 50,000 tons of 3- to 12-inch rounded cobble and gravel.
Bids are due Wednesday on the $500,000 to $1 million renourishment project.
Jon Warrick, a USGS research geologist, said the agency has studied the beach at the mouth of the Elwha River for seven years.
In that time, more than 30 feet of beach has eroded.
“It’s been substantial,” said Warrick, who added that the erosion rate has increased over time.
River mouth
Based on aerial photographs, the erosion at the river mouth was about 2 feet per year from the 1940s to the 1990s.
That rate has since doubled, Warrick said.
Erosion is happening on the east side of the river mouth, not the west side toward Freshwater Bay, because of the angle that waves hit the beach.
Matt Beirne, Lower Elwha environmental coordinator, said tribal elders recall a broader, sandier beach.
“They were able to go clamming,” he said. “Of course, you’re not now.”
Beirne said dam removals will “certainly slow the trend” of habitat loss.
Warrick said it is unclear exactly how the beach will respond to the dams being removed.
“We’ll just have to see,” he said.
“It’s one of the exciting things about this.
“It’s never been done before. We are going to learn a lot along the way.”
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Reporter Rob Ollikainen can be reached at 360-417-3537 or at rob.ollikainen@peninsuladailynews.com.
Reporter Tom Callis contributed to this report.
